Partial Substitution of Copper with Silver in Cu₂ZnSnS₄: An Efficient Strategy to Boost the Performance of Self-Powered Broadband Photodetectors in Superstrate Configuration

Self-powered broadband photodetectors (SPBPDs) hold great potential for next-generation optoelectronic applications, but their performance is often limited by interface defects that impair charge transport and increase recombination losses. In this work, we report the enhancement of the photodetection efficiency of SPBPDs by partially substituting copper (Cu) with silver (Ag) in kesterite Cu₂ZnSnS₄ (ACZTS) thin films. Varying Ag concentrations (0%, 2%, 4%, 6%) are incorporated into the CZTS layer, forming a TiO₂/ACZTS heterojunction in superstrate configuration fabricated via a low-cost sol-gel spin-coating technique with low-temperature open air annealing avoiding conventional postdeposition sulfurization or selenization. Photodetection performance varied significantly with Ag content in CZTS layer, where optimal performance is observed for 4% Ag doping. Under the simulated solar spectrum (100 mW/cm²), the TiO₂/4% ACZTS device demonstrates superior performance with an ON/OFF ratio of 6.0 × 10², a photoresponsivity of 0.42 mA/W, and a detectivity of about 2.5 × 10⁹ Jones. In contrast, under 405 nm incident radiation, the device performance improves significantly, achieving an ON/OFF ratio of approximately 4.6 × 10³, a photoresponsivity of around 63.9 mA/W, and a detectivity of about 4.8 × 10¹¹ Jones which are the highest reported values observed for CZTS-based single-junction superstrate SPBPDs without a crystalline silicon wafer. Ag doping effectively reduces interface defects and enhances carrier dynamics, as evidenced by capacitance-voltage (<i>C</i>-<i>V</i>) and drive-level capacitance profiling (DLCP) measurements of the fabricated heterojunction. This approach offers a novel strategy for enhancing SPBPD performance through partial cation substitution, paving the way for advanced photodetectors in diverse optoelectronic applications.